Hybrid T-junction switch

ABSTRACT

A microwave hybrid T-Junction switch using the two main (symmetrical) arms as input/output ports and two branch arms (E and H-plane) as control terminals, one of the branch arms containing a biased diode for switching and the other branch arm terminated with a fixed electrical short.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to microwave hybrid junctions as switches.

2. Description of the Prior Art

Microwave hybrid junctions are four terminal devices which ideally havethe property that power supplied to a given terminal is divided usuallyequally between two of the remaining terminals with nothing reflectedback or coupled to the fourth terminal. However, by appropriatepositioning of short circuits in said two of the remaining terminals(control terminals), the energy provided to them can reflect to returnto the input or add at the fourth terminal. A selectively forward orreverse biased diode can effect electrical movement of a short circuitlocation so as to switch the path from the given terminal to the fourthterminal between open and closed conditions. A particular advantage ofthis type of arrangement is that the power from the given terminalsplits between the two control terminals whereby a diode located in oneof them never sees more than half the power.

In U.S. Pat. No. 3,559,108 to H. Seidel, coupler switches are disclosedusing both the quadrature and 180° type of hybrid junctions. Seidel usespairs of variable impedances such as a diode for each of the controlterminals. Also for modulation, demodulation, phase inversion orswitching, the symmetrical arms of a T-junction are conventionally usedfor control. The nonsymmetrical arms become the signal in and out ports.This is the kind of configuration Seidel uses as is clear from hisstatement (column 6, lines 72-73), the 180° coupler switch is open inits symmetric state. When using the nonsymmetrical arms for control, asin the present invention, symmetrical impedances in the control armsclose the switch. The asymmetry of the nonsymmetrical arms has made itvirtually impossible to design control elements in those arms from astrictly theoretical approach. As a result the symmetrical arms havealways provided the control function. Using the nonsymmetrical arms forinput/output functions raises problems of packaging, particularly inwaveguide.

For the most compact insertion of a switch in many waveguide systems,the input/output ports are preferably colinear. The symmetrical arms ofa hybrid T-junction are readily colinear while the nonsymmetrical armsare not.

U.S. Pat. No. 3,93l,599 to the present inventor discloses a phaseinvertor using asymmetrical switching in the main arms to produce phaseinversion. As with Seidel, variable impedances are provided at bothcontrol terminals.

SUMMARY OF THE INVENTION

In accordance with the invention, a hybrid T-junction is provided usinga variable impedance in a first branch arm for switching a path throughthe main arms between open and closed conditions. The second branch armis shorted at an electrical distance from the input terminal that eitherprovides the closed condition or the open condition over the broadestfrequency range with the variable impedance in its highest impedancecondition. For the former a low impedance state of the variableimpedance opens the switch. For the latter a low state of the variableimpedance closes the switch. The first branch arm may be a pseudoH-plane arm in the form of a coaxial stub in which the variableimpedance is coaxially positioned.

Thus it is an object of the invention to provide a microwave hybridjunction in which switching is performed by a variable impedance in afirst branch arm with a fixed electrical short in the second branch arm.

Further objects and features of the invention will become apparent uponreading the following description together with the drawing.

BRIEF DESCRIPTION OF THE DRAWING:

FIG. 1 is a diagrammatic illustration of a generalized embodiment of theinvention.

FIG. 2 is a schematic representation of one embodiment of the invention.

FIG. 3 is a schematic representation of a second embodiment of theinvention.

FIG. 4 is an isometric drawing of a device according to FIG. 2.

FIG. 5 is an enlarged detail of a cross-section through 5--5 of FIG. 4.

FIG. 6a is a front elevation of a second device according to FIG. 2.

FIG. 6b is a top plan view of the device of FIG. 6a.

FIG. 6c is a left side elevation of the device of FIG. 6a.

FIG. 6d is a right side elevation of the device of FIG. 6a.

FIG. 6e is a bottom plan view of the device of FIG. 6a.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

A microwave hybrid T-junction, popularly called a Magic Tee, is a fourterminal pair coupler having two symmetrical arms herein called "mainarms" and two nonsymmetrical arms herein called "branch arms". Inwaveguide, one of the branch arms is an E-plane (series) arm and theother is an H-plane (shunt) arm.

A generalized embodiment of the invention is depicted diagrammaticallyin FIG. 1 with the main arms of magic tee 10 represented by arms 12 and13, the E-plane arm represented by shorted arm 14 and the H-plane armdepicted by shorted arm 11 with switch 15 arranged so as to change theelectrical length of terminal pair 11 by one quarter wavelength at thedesign frequency.

One preferred embodiment is depicted electrically in FIG. 2 andmechanically by FIGS. 4 and 5. In each of the figures, the samedesignation numbers are used for the respective arms of the couplerdespite differences in the embodiments.

In FIGS. 2-6, arm 11 is a pseudo H-plane arm. Magic Tee coupler 20 ismade of waveguide as depicted in FIG. 4 with shorted coax-to-waveguidetransition 16 serving as the pseudo H-plane arm. Transition 16 ispositioned in the broad wall of coupler 20 facing the central axis ofE-plane arm 14. PIN diode 17 (FIGS. 2 and 5) substitutes for the centralconductor of the coax and has first electrode 18 connected to end 21 oftransition 16. Second electrode 22 of diode 17 is connected to probe 38which extends into E-plane arm 14. Lead 24 connects probe 38 through anarrow wall of arm 14 to switch 25. Switch 25 selectively connectsdirect current source 26 across diode 17 to bias it into a conductivecondition. It will be understood that switch 25 may also be a two poleswitch and a second direct current source of opposite polarity may beconnected to the other pole for reverse biasing diode 17. This issometimes desirable or even necessary depending, for example, on thecharacteristics of diode 17 and the energy being propagated.

In coupler 20 arm 14 is terminated with short circuit 27 that is adistance from the center of junction 20 that matches the characteristicsof transition 16 so as to provide maximum reflection to arm 12 of energyintroduced at arm 12 over the greatest bandwidth about the designfrequency with switch 25 open. In this condition, energy at the designfrequency introduced at arm 12 splits evenly between the branch arms 11and 14 and is totally reflected in phase toward arm 12 and antiphasetoward arm 13. Thus there is no output at arm 13. Closing switch 25changes the phase of wave energy reflected in arm 11 by 180°. Asconventional in the art, minor adjustments of the structure oftransition 16 will commonly be required to obtain accurate 180°switching in any given prototype. Energy at the design frequency nowintroduced at arm 12 splits evenly between the branch arms 11 and 14 andis totally reflected in phase toward arm 13, thus adding to propagateout arm 13.

A variation is depicted in FIG. 3. FIG. 3, arm 14, is electrically shortcircuited by short 31 a distance from the center of junction 30 thatmatches the characteristics of transition 16 so as to provide maximumtransfer of energy from arm 12 to arm 13 over the greatest bandwidthabout the design frequency with switch 25 open. Diode 17 is replaced bytwo diodes 32 in arm 11. The use of two diodes permits doubling thepower since the energy will normally divide evenly between the twodiodes. The diodes must either be matched or biased through separateimpedances to prevent one diode from "hogging" the bias current. A largenumber of diodes may be used for increasing the power handling abilityfurther. In the configuration of FIG. 3 energy at the design frequencyintroduced at arm 12 will be reflected from the branch arms in phasetoward arm 13 when switch 25 is open. Conversely, closing switch 25 willcause the energy to cancel toward arm 13 providing no output at arm 13.It will be seen that by changing the length of arm 14 energy can be madeto pass through with a forward bias of the diodes or it can be blockedwith a forward bias of the diodes. The choice in any given situation canbe determined to provide minimum stress on the diodes or preferentialband characteristics. It has been found that the band characteristicscan usually be balanced so as to be almost identical for both conditionsof diode biasing, but this is not always the most desirable. The diodes,in some systems are more likely to break down in their forward conditionwhile in other systems they break down more readily in their reversecondition. The inventive junction can be selected to favor the lessreliable condition of the diodes for the particular system.

While the pseudo H-arm arrangement has been found to work particularlywell, the same basic principles apply to arrangements using aconventional H-arm with a conventional waveguide to coax transition ineither the H-plane or E-plane arms. Whichever arm contains thetransition, the other arm is shorted or opened in the manner describedabove.

FIG. 4 depicts an actual device following the schematic of FIG. 2 butomitting switch 25 and current source 26. The device of FIG. 4 isidentical to that of the basic magic tee waveguide coupler with theexception that the usual H-plane arm has been replaced by waveguide tocoax transition 36 acting as a pseudo H-arm plane arm. E-plane arm 14 iseffectively short circuited at its open end by metal strip 27 connectedbetween its broad walls midway between the two narrow walls. Waveguideto coax transition 36 terminates inside the coupler with probe 38designed in a conventional manner for a waveguide to coax transition.Performance has been found best with probe 38 extending slightly intobranch arm 14. Lead 24 from probe 38 is taken out through narrow wall 39of arm 14 through choke/insulator 41 inserted in wall aperture 43.

Since it is difficult to calculate the exact electrical length of ashorted waveguide to coax transition at a specific design frequency, amodel with adjustable parameters is best used as a prototype for anyspecific embodiment in accordance with the invention. An easy variablefor the prototype is a telescoping arm 14 or a short 27 that may bemoved along the arm axis. A first approximation can be made with theshort at or near the opening into branch arm 14 from main arms 12 and13. This is so because transition 36 has little electrical length in theconfiguration of FIG. 4.

FIG. 5 is a cross-section through the center of FIG. 4 detailingwaveguide to coax transition 36. Diode 17 is a tiny element held inplace by screw cap 42 threaded into body 52 of coupler 20 underneathtransition 36. The other end of diode 17 is supported bypolytetrafluoroethylene washer 48. Washer 48 is supported in couplerbody 52 by metal cylinder 50 extending into the waveguide cavity. Metalprobe 38 extends across the main arm cavity into E-arm 14. When usingmore than one diode, identical transitions can be lined up side by sidetransversely to the main arm axis. Also a plurality of diodes may beinstalled in the same transition since the diode elements themselves arequite tiny.

One of the advantages of the inventive coupler is that it can beconveniently adapted to various compact configurations. FIG. 6a-6edepict a configuration in which the length of the main arms isdiminished to virtually nothing while E-arm 14 and pseudo H-arm 11 areboth incorporated in a flat metal slab. The components coupled to themain arm ports then become, in effect, the main arms. Aperture 56 wasdrilled through first edge 57 located centrally relative to thethickness of plate 55. Rectangular aperture 58 matching thecross-section dimensions of a desired waveguide is machined throughplate 55 intersecting aperture 56. Second edge 60 is the edge oppositeedge 57 while third and fourth edges 63 and 64 are the edges transverseto edges 57 and 60. Aperture 58 is located in the center of the widthdimensions of plate 55 between edges 63 and 64. Second rectangularaperture 61, also matching the dimensions of desired waveguide, ismachined from top edge 60 to intersect with apertures 56 and 58. Secondhole 62 is drilled in from side 64 above the plane of aperture 58 andintersecting with aperture 61. Hole 56 is threaded and a waveguide tocoax transition as depicted in FIG. 5 is inserted. Cylindrical support50 enters into the aperture 58 a short distance. Probe 38 extendsfurther entering into aperture 61. Lead 24 connected to probe 38 passesthrough teflon plug 65 fitted into hole 62. Shorting bar 67 ispositioned across the narrow dimension of aperture 61 to serve as ashort.

The inventive concept is readily adapted to a number of configurationsvarying from those described but coming within the inventive concept. Inall of these particular advantage is derived from the fact that thevariable impedance used to switch the coupler, switches all the powerwhile handling only half the power. The variable impedance may be asimple conductive pin in the position of the diodes disclosed andoperated by a solenoid to provide the same function as the diodesdescribed.

While the embodiments depicted include a showing of variable impedanceelements (diodes) in parallel, they can also be used in series towithstand higher potential levels. It is of particular significance thatthe invention provides junction structures with no limitation on thelength of the main arms other than imposed by the thickness of the metalfrom which the junction is constructed. This gives a great latitude inselection of packaging configurations.

It should also be recognized that, although the described embodimentsuse mechanical shorts in one of the branch arms, the same effect can beobtained by terminating the arm with a waveguide to coax transition.Such a transition allows the use of a mechanical open which reflects anelectrical short a distance away. In this way an open termination may beused in place of a short.

Accordingly, it is intended to cover the invention as set forth in theappended claims.

I claim:
 1. A microwave hybrid T-junction in which energy introduced atone of the main arms is blocked or passed to the other main arm bychanging the state of a variable impedance in one of the branch armscomprising:(a) a waveguide body having a first main arm, a second mainarm, a first branch arm and a second branch arm, one said branch armbeing an E-plane arm and the other being an H-plane arm; (b) a variableimpedance located in said first branch arm variable between a highimpedance state and a low impedance state by application of anelectrical control signal, said variable impedance arranged so as tochange the phase of wave energy reflected in said first branch arm atthe design frequency by 180 degrees when said variable impedance changesimpedance states; (c) a short circuit of conductive material across saidsecond branch arm at a location providing a maximum energy transfercondition between the first and second main arms at a design frequencybandwidth for one condition of said variable impedance.
 2. A microwavehybrid T-junction according to claim 1 wherein said variable impedanceis a diode mounted in a waveguide to coaxial line transition structure.3. A microwave hyrid T-junction according to claim 1 wherein said firstbranch arm is a pseudo H-plane arm in the form of a waveguide to coaxialline transition element and said variable impedance is a diode mountedcoaxially within said element.
 4. A microwave hybrid T-junctionaccording to claim 3 wherein said diode has a first electrode connectedto a short circuited end of said element and a second electrodeconnected to a probe extending partially into said second branch arm. 5.A microwave hybrid T-junction according to claim 4 further comprising anelectrical control signal source, a first connection from said sourcethrough a wall of said second branch arm to said probe and a secondconnection from said source to the body of said coupler so as to form acircuit with said diode.
 6. A microwave hybrid T-junction according toclaim 3 wherein said waveguide body is made in the form of a flat metalplate incorporating said first branch arm and said second branch armwithin the periphery of the plate.
 7. A microwave hybrid T-junctionaccording to claim 1 wherein said variable impedance comprises at leasttwo variable impedance elements.
 8. A microwave hybrid T-junctionaccording to claim 1 wherein the length of the main arms issubstantially the thickness of the metal from which the junction isconstructed.